The rare earth elements are the series of elements that extends from lanthanum to lutetium on the periodic table. Yttrium and scandium are sometimes included in the rare earth element grouping. These elements are very valuable and in some cases in short supply.
The processing of rare earths is dominated by China. The history of rare earth extraction is well documented in a textbook “Extractive Metallurgy of Rare Earths”, by C. K. Gupta (Author), N. Krishnamurthy CRC Press (2004). Two of the major deposits of rare earths are the Bayan Obo deposit in China and the Mountain Pass deposit in California. These deposits contain bastnasite as the major rare earth mineral. The percentage of the total rare earth content of the bastnasite ores contained as individual rare earth elements is dominated by the light rare earths (La, Ce, Pr, Nd, Sm) with low content of heavy rare earths (Eu—Lu, Y).
The recovery of rare earths from mineral deposits is complicated and costly. For example at both Bayan Obo and Mountain Pass, the ore is crushed and ground to fine size (150 mesh or 104 micrometers in diameter) and then subjected to a complicated physical and chemical separation process. The finely ground Mountain Pass ore (Gupta and Krishnamurthy) has historically been treated with 6 stages of conditioning with chemicals such as soda ash, fluorosilicate, distilled tall oil, ammonium lignin sulfonate and steam for heating, followed by froth flotation to make a mineral concentrate. The mineral concentrate is then subjected to chemical steps (acid washing) and high temperature calcination before refining. A similar process has been applied at Bayan Obo with the addition of steps of magnetic separation and gravity concentration (separation based on differential magnetic properties) to produce a mineral concentrate.
The mineral concentrate produced by beneficiation is then typically chemically attacked using acid (eg. sulfuric acid) or base (eg. sodium hydroxide) to decompose the rare earth minerals and allow subsequent extraction to an aqueous solution. The extracted rare earths may then be purified by various chemical methods. Finally rare earths may be separated by the process of multi-stage solvent extraction to produce individual rare earth elements of high purity for commercial use. The process of solvent extraction is for example reviewed in: Xie, F., Ting, T. Z., Dreisinger, D. B., Doyle, F., “A Critical Review on Solvent Extraction of Rare Earths from Aqueous Solutions”, Minerals Engineering (2014), 56, 10-28.
Additional aspects of selected rare earth extraction methods are described in: Dreisinger et al., “The Processing of REE's from Search Minerals Foxtrot Resource”, Proceedings of Rare Earths 2012, Eds, J. R. Goode, G. Moldoveanu, M. S. Reyat, CIM Metsoc (Montreal), 81-94. This paper outlines a process of beneficiation to produce a mineral concentrate. The techniques of gravity, flotation and magnetic separation are used to upgrade a rare earth ore containing various rare earth minerals. The finely ground concentrate is then acid treated to convert the rare earth minerals to acid soluble form. The rare earth sulfates are then water leached. The leachate is then purified and the rare earth elements recovered by an oxalate precipitation process.